The present invention relates generally to methods and systems for producing a plurality of different pore size microporous phase inversion membrane each having any one of a plurality of different pore sizes from a single master dope batch. More specifically, it relates to methods and systems for selectively essentially instantaneously thermally manipulating at least a portion of a master dope batch to a temperature within about -0.2 to about -0.15.degree. C. of a predetermined temperature which has proven to yield microporous phase inversion membrane having about a specific pore size formed therein when processed. Most specifically, it relates to methods and systems for exacting essentially instantaneous thermal manipulation to a finer degree of control over a small portion of dope incrementally processed from a master batch such that a wider range of possible pore sizes can be selectively formed in microporous phase inversion membrane produced therefrom than was previously believed possible from a single master batch of dope and in a short time frame.
Microporous phase inversion membranes are well known in the art. Microporous phase inversion membranes are porous solids which contain microporous interconnecting passages that extend from one surface to the other. These passages provide tortuous tunnels through which the liquid which is being filtered must pass. The particles contained in the liquid passing through a microporous phase inversion membrane become trapped on or in the membrane structure effecting filtration. A slight pressure, generally in the range of about five (5) to about fifty (50) psig (pounds per square inch gauge) is used to force fluid through the microporous phase inversion membrane. The particles in the liquid that are larger than the pores are either prevented from entering the membrane or are trapped within the membrane pores. The liquid and particles smaller than the pores of the membrane pass through. Thus, a microporous phase inversion membrane prevents particles of a certain size or larger from passing through it, while at the same time permitting liquid and particles smaller than that certain size to pass through. Microporous phase inversion membranes have the ability to retain particles in the size range of from about 0.01 to about 10.0 microns.
Many important micron and submicron size particles can be separated using microporous membranes. For example, red blood cells are about eight (8) microns in diameter, platelets are about two (2) microns in diameter and bacteria and yeast are about 0.5 microns or smaller in diameter. It is possible to remove bacteria from water by passing the water through a microporous membrane having a pore size smaller than the bacteria. Similarly, a microporous membrane can remove invisible suspended particles from water used in the manufacture of integrated circuits in the electronics industry. Microporous membranes are characterized by bubble point tests, which involve measuring the pressure to force either the first air bubble out of a fully wetted phase inversion membrane (the initial Bubble Point, or "IBP"), and the higher pressure which forces air out of the majority of pores all over the phase inversion membrane (foam-all-over-point or "FAOP"). The procedures for conducting initial bubble point and FAOP tests are discussed in U.S. Pat. No. 4,645,602 issued Feb. 24, 1987, the disclosure of which is herein incorporated by reference. The procedure for the initial bubble point test and the more common Mean Flow Pore tests are explained in detail, for example, in ASTM F316-70 and ANS/ASTM F316-70 (Reapproved 1976) which are incorporated herein by reference. The bubble point values for microporous phase inversion membranes are generally in the range of about five (5) to about one hundred (100) psig, depending on the pore size and the wetting fluid.
Methods and Systems for preparing the dope used to produce microporous membrane are known in the art. There are numerous methods of preparing the dope. A number of the known prior methods of dope preparation are discussed in representative U.S. Pat. No. 3,876,738 issued Apr. 8, 1975, U.S. Pat. No. 4,340,480 issued Jul. 20, 1982, U.S. Pat. No. 4,770,777 issued Sep. 13, 1988, and U.S. Pat. No. 5,215,662 issued Jun. 1, 1993, the disclosure of each is herein incorporated by reference.
One specific method for the preparation of dope (U.S. Pat. No. 3,876,738) to produce a specific pore size when processed into microporous membrane was to batch formulate the dope by polymer to nonsolvent to solvent ratio as a predictive control of pore size. Batch formulation was conducted at an assumed maximum temperature. In practice, to maintain a single precise mixing temperature over a four (4) to six (6) hour period necessary to compete the mixing cycle is very difficult. Precision in formulation and precision in the uniformity of mixing (shear history and temperature history) are equally important to the successful commercialization of phase inversion membrane having specific and controlled pore size formed therein.
During the mixing of the dope ingredients, solvent and nonsolvent were mixed first and then the polymer was added to the mixture of nonsolvent with the temperature being controlled and assumed not to exceed a certain temperature. In this formulation process, the solvent, such as, for example, formic acid, was first placed in a vessel. Next, the nonsolvent, such as, for example, methanol, was added and the nonsolvent and solvent were allowed to react and reach equilibrium. After the solvent and nonsolvent mixture reached equilibrium, the polymer, such as, for example, nylon was added and blended with the solvent and nonsolvent mixture for a sufficient amount of time and under reasonably controlled conditions of temperature and solution agitation (shear) to effect the dissolving of the nylon polymer in the solvent/non-solvent mixture until the polymer/solvent/nonsolvent mixture reached equilibrium.
It is known that processing relatively large bodies of dope, such as that used in the production of microporous phase inversion membranes, is accompanied by many difficulties such as the need to formulate separate dope batches for each size pore phase inversion membrane produced as well as the problems in controlling the temperature of the dope during the batching process.
Dope that has been formulated according to a particular formulation may, due to process variables, produce out of specification phase inversion membrane. In the past, in order to salvage out of specification batches, the out-of-specific batches were reprocessed by bulk heating to a higher temperature which produced a larger pore size when reprocessed. Dope reprocessing included elevating the dope temperature of large amounts of dope such as, for example, up to one hundred (100) gallons and even larger batches to a predetermined target temperature.
Dope reprocessing was needed because, during the batch formulation process, formulation errors were introduced into the batch such as incorrect amounts of ingredients, different nylon batches and other processing errors that occurred in the batch mixing process. Because of the differences in the resulting dope batches due to different lots of nylon, different types of reactants and etc., microporous phase inversion membrane having the same exact pore size was not always produced from different batches prepared according to the same recipe each and every time. In fact, a band of predictable pore sizes for each specific formulation was developed over a period of time.
If the pore size from a particular batch of a particular formulation turned out to be too open or have larger than the maximum pore size permitted by the specification for the end use, then that batch was scrapped, due to production schedules as retaining the dope for a future run at that pore size was impractical and because it was not possible to reprocess the batch to produce phase inversion membrane having a smaller pore size. If the formulation of a specific dope batch resulted in the characterization of the pore size being too tight or small, then that batch of dope was reprocessed by batch bulk reheating.
It should be pointed out that during production runs of microporous phase inversion membrane, it is important to produce microporous phase inversion membrane having the desired pore size and/or pore size distribution.
As described above, in the past, out-of-specification dope for the production of microporous phase inversion membrane was conventionally reprocessed by bulk reheating of the dope in a vessel under pressure having an external water jacket and an internal agitating means to correct missed values. As is known, heat transfer by bulk heating to a large mass of material, such as a dope batch undergoing specification correction/reprocessing, utilizing the thermal transfer jacket and conventional agitation means has proven to be difficult. As is known, this method of reprocessing dope can produce areas within the vessel where fluid flow is reduced or stagnant and, thus, the dope in those areas of the vessel may not be sufficiently intermixed with the entire mass of dope to ensure that the entire mass of dope was elevated to about the same temperature. If some portion of the dope batch being reprocessed was heated above or had already been heated above the target temperature, then that portion of the dope when processed produced pores in the microporous phase inversion membrane that are larger than desired. The continued, prolonged mixing of these portions does not necessarily result in a uniform dope of narrow pore size distribution, but, may in fact, have the opposite effect of increasing (widening) the pore size distribution resulting in an inferior phase inversion membrane.
Specifically, it may be that each portion of the dope in the batch was not heated to the target temperature but may, in fact, have been heated to a temperature either higher or lower than the target temperature. For example, portions of the dope that for one reason or the other remained closer to the internal wall of the external heating jacket of the vessel during reprocessing tended to be heated to a higher temperature than the portions of the dope which do not come in contact with the internal wall of the heating jacket around the large mass of dope contained inside the vessel.
A temperature control problem was identified during the reprocessing of conventionally formulated dope batches undergoing reprocessing in that not all portions of the dope had been heated to the new temperature within a very tight temperature range. Specifically, it is now believed that the portions of the dope proximate the inner wall of the heating jacket of the vessel were heated to a temperature above the new target temperature during reprocessing and, thus, when cast, produced microporous phase inversion membrane having pores larger than desired as well as an unacceptable pore size distribution.
In summary, in this batch formulation process, the dope formulation (solvent, nonsolvent, polymer ratio) was key to controlling pore size in the microporous phase inversion membrane. Using the batch formulation method as a predictive control of pore size in microporous phase inversion membrane, microporous phase inversion membrane having a specific pore size was produced from a specifically formulated dope batch.
Another prior method of making dope (U.S. Pat. No. 4,340,480) to produce micropourous phase inversion membrane comprised mixing a dope to a maximum nonsolvent level concurrently to a point in fact where so much nonsolvent is being added that the system started to kick out and precipitate the polymer. The non-solvent was added to the mixture in a very high sheer region. By using this method, it was claimed that the pore size of the membrane produced could be controlled on a batch basis by controlling the mixer speed. Specifically, the dope is formulated by first mixing formic acid and nylon, then introducing water in a very high sheer region and finally adjusting the speed of the mixer. This method appears to correlate pore size with mixer speed but does not appear to either measure or attempt to control temperature. As is known, there are more precise ways for controlling temperature than trying to control the impeller speed of a mixer.
Another prior, specific method for the preparation of dope using nylon 46 (U.S. Pat. No. 5,215,662) for producing a specific pore size when processed into microporous membrane was to mix a greater proportion of the nylon 46 polymer in a solvent/nonsolvent solution to produce smaller pores in the resulting membrane. In this method, nylon 46 was slowly added into the mixing solvents and nonsolvents at temperatures ranging from about 25.degree. C. to about 80.degree. C. at a speed sufficient to prevent the polymer from clumping, but insufficient to cause overheating and polymer degradation (the only apparent process temperature control parameter mentioned). As described in the patent, within this temperature range, higher temperatures caused dissolution to proceed more rapidly and the mix time to total dissolution can be decreased. In this patent, higher solution temperatures were purported to result in somewhat larger pore sizes and temperature controls were purported to be used to further manipulate the pore sizes of the produced membrane, in connection with variations in the composition of the bath (See Example 4). However, there appears to be no attempt to precisely control the temperature of the solution during formulation.
This patent appears to teach the use of a dispersion system, which included temperature controls, preferably a heat exchanger, to change the pore size and the viscosity of the mixture as necessary to obtain a smooth, even flowing of the mixture for processing into membrane. According to the patent, as the temperature of the mixture rises, and as the higher temperatures are maintained for longer periods of time, membrane pore size was increased. This feature was purported to allow production flexibility because the solutions temperature reportedly could be manipulated to produce a range of pore sizes from a single batch of solution. Further, the composition and process temperature control manipulation supposedly enabled continuous production of the material with mixed or variable pore size and distribution from a single batch of nylon 46 solution.
As shown in example 4 of the patent, it appears that it was the heat exchanger combined with a bath having a different composition that was actually used to increase the pore size of the membrane produced from the solution batch and not thermal manipulation alone. While at least a part of the resulting pore size increase was attributed to the temperature increase, how much of the increase, if any, was due to the temperature increase or to the change in the bath composition is not discernible from the patent. Specifically, the patent teaches that smaller pore size material results from higher proportions of solvent in the bath. In Example 4, the cause of the resulting pore size increase is ambiguous at best, since the proportion of solvent in the bath was reduced from thirty two percent (32%) to twenty two percent (22%), in accordance with the previously known teaching for increasing pore size.
As described above, thermal manipulation to change the pore size in a membrane produced from a dope has long been recognized and has been used in reprocessing out of specification dope, as discussed above. However, this recognized property of the dope was dependent on raising the temperature of the dope to a temperature higher than that to which the dope had previously been processed. While this patent discusses controlling the process temperature as one factor in enabling continuous production of material with fixed or variable pore size from a single batch of nylon 46 solution, it fails to provide any specific temperatures other than a wide temperature range. Further, in the only example relative to varying pore size, the patent combines process temperature manipulation with the composition of the dope and the composition of the bath to effectuate the pore size change but only in one direction, from smaller to larger. There was no apparent effort to control the temperature of the solution at a specific temperature or any effort to try to lower the temperature of the solution to produce a smaller pore size.
Following the teachings of this particular patent, using thermal manipulation to change the pore size and viscosity of the mixture, as the solution is heated to higher temperatures, the viscosity of the dope becomes such that it might not be usable in a solution casting operation, unless controlled. Specifically, as the particular solution is heated to higher temperatures, processing problems will most likely be encountered including those related to viscosity, degassing of volatile components, foam formation and quenching problems, without adequate viscosity control.
The methods taught in this patent are not applicable to Marinaccio style Nylon 66 dopes and the membrane products produced therefrom, for the following reasons: 1) the patent is directed toward attempting to produce a skinned membrane, with a radically altered pore structure just below the qualifying skin layer. In this method, the quality and integrity of the skinned membrane is completely dependent on the quality of the first few microns of surface thickness. With this method, even the smallest imperfection (air entrapment, substrate fiber breach, etc.) in the skin will destroy the integrity of the product. For this reason, the methods disclosed in the patent must restrict the casting solution viscosity to a very narrow practical range, to ensure wetting of the substrate, minimization of entrapped air, and "smooth, even coating of the mixture", to ensure the integrity of the finished membrane product. There is, however, a practical limit to the solution viscosity; therefore a single stage thermal treatment and hot casting would potentially lower the viscosity to an impractical point, thus limiting the useful range of resultant pore sizes. 2) Additionally, the single stage thermal treatment and hot casting would be harmful to the resulting product, in that the volatile non-solvent components of the Marinaccio style dope (Methanol and Methyl Formate) will de-gas in an uncontrolled manner upon casting at a temperature above 34.degree. C. (boiling point of Methyl Formate), and form bubbles, voids and other imperfections in the surface and matrix of the membrane. These voids are not desirable in commercial micropourous membrane.
In the end, the teaching of this patent appears ambiguous as to the effect of temperature alone on pore size because smaller pore size materials could result primarily from, 1) different casting dope solution formulations, or 2) higher proportions of solvents in the bath as it was known that a range of different pore sizes could be produced from a single solution by changing the proportions of solvents in the bath.
In summary, the prior art can be described as a non-real time predictive batch-type process that uses formulation to initially control pore size and bulk reheating as a predictive thermal manipulation to produce a predictive pore size to correct an improperly formulated batch, or improperly controlled initial mix cycle, sheer speed control to introduce the nonsolvent in the preparation of the dope as a batch of liquid to be processed into a membrane and bath solvent control in order to vary the pore size. In some prior art, discussed above, at the end of the formulation process, the dope had a viscosity related to the process temperature. There was no apparent attempt to independently control the viscosity of the dope prior to moving the dope to a membrane production apparatus.
One possible approach for solving the temperature control problem during dope batch formulation would be to precisely control the formulation of a single batch at a low temperature, less than the maximum temperature usually seen during the formulation of some specific batch formulations, for producing a specific pore size while maximizing the non-solvent to solvent ratio.
Since the formulation of different dope batches for each specific pore size microporous membrane being produced resulted in a considerable amount of the resulting microporous membrane being placed in inventory, systems and methods for producing any one of a plurality of specific pore sizes from a single master dope batch would be desirable. Such systems and methods should provide for the formulation of the master dope batch at a temperature equal to or below the target temperature for the smallest pore size of the possible plurality of pore sizes to be produced from the single master dope batch. Such systems and methods should provide for the incremental elevation of selected portions of the single master dope batch to any one of a plurality of target temperatures such that microporous membrane having any one of a plurality of corresponding pore sizes could be sequentially produced from a single master dope batch. Such systems and methods should provide for the temperature control of at least a portion of the single master dope batch to about -0.2.degree. C. of a target temperature prior to that portion at the target temperature being transferred to the microporous membrane casting step. Such systems and methods should provide for the accurate control of the temperature seen by substantially all of that portion of the dope to about -0.15.degree. C. prior to that portion of the dope being transferred to the microporous membrane casting step. Such systems and methods should eliminate the necessity for preparing a dope batch according to individual unique formulations for each pore size, thus resulting in significant cost savings and flexibility in the usage of dope batches. Such systems and methods should also provide the ability to selectively change the pore size of the microporous membrane being produced from a master batch after a certain amount of microporous membrane has been produced at one pore size and begin producing microporous membrane at another pore size utilizing the same master dope batch, resulting in significant cost savings and reduction of inventory of microporous phase inversion membrane produced.